CBSE Class 6 Answered

The distance travelled by a body in 'n' seconds is, 

S_n = u + (a/2) × (2n - 1)

 S₃ = u + (a/2) × (5)

We know, S₃ = 4 

4 = u + (5a/2) ... (1) (here n = 3) 

S₅ = u + (a/2) × (9) (here n = 5) 

We know, S₅ = 12

12 = u + (a/2) × (9) ... (2)

On solving equations (1) and (2),  we get

u = - 6 m/s

and a = 4 m/s²

Thus, distance covered in 8 s, 

S₁ = - 6 × 8 + 1/2 × 4 × 25 = 80 m 

Distance covered in 5 s, 

S₂ = -6 × 5 + (1/2) × 4 × 25 = 20 m 

Thus, the distance travelled in the next 3 seconds,

= 80 m - 20 m = 60 m